I'm proud to introduce you to... Arduway, a robot made of Arduino and Lego NXT components.I'm releasing soon a software library to control LEGO NXT motors and a few sensors I have used: it's not as well documented as it could but I think it will still be better than nothing...

The drift is not evident in the first moments of functioning so it was satisfying enough for our purpose.I tried anyway to correct it (to make the robot stand up hours and hours) using a band-pass filter (instead of an integrator) but it was not working as well as one would expect....Are you aware of any other method?

It would certainly be better not to have to integrate: this would avoid the integration drift.

As a conclusion... if you're thinking of creating your own Arduway think of replacing the gyroscope with a compass that directly returns the angle.

To do it right, it sounds like you need an accelerometer, too. David Anderson provides a pretty detailed description of how he did this on his nBot web site

He explains, "The gyroscope and accelerometer are combined with complementary filters to provide an inertial reference sensor. The ADXL202 accelerometer provides accurate static tilt information, when the robot is not accelerating. The gyroscope can be integrated to provide accurate dynamic tilt information, but the integration tends to drift over time. Combining the two sensors provides a robust inertial measurement."

But, there are apparently a number of different ways to make a balancing robot. I've heard of at least one person who managed to do it using distance sensors to figure out the tilt based on the distance of the sensor to the ground.

In my opinion an angle sensor can be enough (it certainly is enough in Matlab simulations).... and it is certainly less expensive than two sensors (BTW, I'm saying that because cheapness was one of the aim of the project).

The robot works pretty well even with only a gyroscope... if you start your own project keep in mind that an angle sensor (or maybe gyro + accelerometer) is better.